The most profound bonds between people begin in our bodies with imitation and synchronized movements.

What intrigues me most about laughter is how it spreads. It’s almost impossible not to laugh when everybody else is. There have been laughing epidemics, in which no one could stop and some even died in a prolonged fit. There are laughing churches and laugh therapies based on the healing power of laughter. The must-have toy of 1996—Tickle Me Elmo—laughed hysterically after being squeezed three times in a row. All of this because we love to laugh and can’t resist joining laughing around us. This is why comedy shows on television have laugh tracks and why theater audiences are sometimes sprinkled with “laugh plants”: people paid to produce raucous laughing at any joke that comes along.

The infectiousness of laughter even works across species. Below my office window at the Yerkes Primate Center, I often hear my chimps laugh during rough-and-tumble games, and I cannot suppress a chuckle myself. It’s such a happy sound. Tickling and wrestling are the typical laugh triggers for apes, and probably the original ones for humans. The fact that tickling oneself is notoriously ineffective attests to its social significance. And when young apes put on their play face, their friends join in with the same expression as rapidly and easily as humans do with laughter.

Shared laughter is just one example of our primate sensitivity to others. Instead of being Robinson Crusoes sitting on separate islands, we’re all interconnected, both bodily and emotionally. This may be an odd thing to say in the West, with its tradition of individual freedom and liberty, but Homo sapiens is remarkably easily swayed in one emotional direction or another by its fellows.

This is precisely where empathy and sympathy start—not in the higher regions of imagination, or the ability to consciously reconstruct how we would feel if we were in someone else’s situation. It began much more simply, with the synchronization of bodies: running when others run, laughing when others laugh, crying when others cry, or yawning when others yawn. Most of us have reached the incredibly advanced stage at which we yawn even at the mere mention of yawning—as you may be doing right now!—but this is only after lots of face-to-face experience.

Yawn contagion, too, works across species. Virtually all animals show the peculiar “paroxystic respiratory cycle characterized by a standard cascade of movements over a five- to ten-second period,” which is the way the yawn has been defined. I once attended a lecture on involuntary pandiculation (the medical term for stretching and yawning) with slides of horses, lions, and monkeys—and soon the entire audience was pandiculating. Since it so easily triggers a chain reaction, the yawn reflex opens a window onto mood transmission, an essential part of empathy. This makes it all the more intriguing that chimpanzees yawn when they see others do so.

Yawn contagion reflects the power of unconscious synchrony, which is as deeply ingrained in us as in many other animals. Synchrony may be expressed in the copying of small body movements, such as a yawn, but also occurs on a larger scale, involving travel or movement. It is not hard to see its survival value. You’re in a flock of birds and one bird suddenly takes off. You have no time to figure out what’s going on: You take off at the same instant. Other­wise, you may be lunch.

Or your entire group becomes sleepy and settles down, so you too become sleepy. Mood contagion serves to coordinate activities, which is crucial for any traveling species (as most primates are). If my companions are feeding, I’d better do the same, because once they move off, my chance to forage will be gone. The individual who doesn’t stay in tune with what everyone else is doing will lose out like the traveler who doesn’t go to the restroom when the bus has stopped.

The herd instinct produces weird phenomena. At one zoo, an entire baboon troop gathered on top of their rock, all staring in exactly the same direction. For an entire week they forgot to eat, mate, and groom. They just kept staring at something in the distance that no one could identify. Local newspapers were carrying pictures of the monkey rock, speculating that perhaps the animals had been frightened by a UFO. But even though this explanation had the unique advantage of combining an account of primate behavior with proof of UFOs, the truth is that no one knew the cause except that the baboons clearly were all of the same mind.

Finding himself in front of the cameras next to his pal President George W. Bush, former British prime minister Tony Blair—known to walk normally at home—would suddenly metamorphose into a distinctly un-English cowboy. He’d swagger with arms hanging loose and chest puffed out. Bush, of course, strutted like this all the time and once explained how, back home in Texas, this is known as “walking.” Identification is the hook that draws us in and makes us adopt the situation, emotions, and behavior of those we’re close to. They become role models: We empathize with them and emulate them. Thus children often walk like the same-sex parent or mimic their tone of voice when they pick up the phone.

How does one chimp imitate another? Does he identify with the other and absorb its body movements? Or could it be that he doesn’t need the other and instead focuses on the problem faced by the other? This can be tested by having a chimpanzee show another how to open a puzzle box with goodies inside. Maybe all that the watching ape needs to understand is how the thing works. He may notice that the door slides to the side or that something needs to be lifted up. The first kind of imitation involves reenactment of observed manipulations; the second merely requires technical know-how.

Thanks to ingenious studies in which chimps were presented with a so-called ghost box, we know which of these two explanations is correct. A ghost box derives its name from the fact that it magically opens and closes by itself so that no actor is needed. If technical know-how were all that mattered, such a box should suffice. But in fact, letting chimps watch a ghost box until they’re bored to death—with its various parts moving and producing rewards hundreds of times—doesn’t teach them anything.

To learn from others, apes need to see actual fellow apes: Imitation requires identification with a body of flesh and blood. We’re beginning to realize how much human and animal cognition runs via the body. Instead of our brain being like a little computer that orders the body around, the body-brain relation is a two-way street. The body produces internal sensations and communicates with other bodies, out of which we construct social connections and an appreciation of the surrounding reality. Bodies insert themselves into everything we perceive or think. Did you know, for example, that physical condition colors perception? The same hill is assessed as steeper, just from looking at it, by a tired person than by a well-rested one. An outdoor target is judged as farther away than it really is by a person burdened with a heavy backpack than by one without it.

Or ask a pianist to pick out his own performance from among others he’s listening to. Even if this is a new piece that the pianist has performed only once, in silence (on an electronic piano and without headphones on), he will be able to recognize his own play. While listening, he probably re-creates in his head the sort of bodily sensations that accompany an actual performance. He feels the closest match listening to himself, thus recognizing himself through his body as much as through his ears.

The field of “embodied” cognition is still very much in its infancy but has profound implications for how we look at human relations. We involuntarily enter the bodies of those around us so that their movements and emotions echo within us as if they’re our own. This is what allows us, or other primates, to re-create what we have seen others do. Body mapping is mostly hidden and unconscious, but sometimes it “slips out,” such as when parents make chewing mouth movements while spoon-feeding their baby. They can’t help but act the way they feel their baby ought to. Similarly, parents watching a singing performance of their child often get completely into it, mouthing every word. I myself still remember as a boy standing on the sidelines of soccer games and involuntarily making kicking or jumping moves each time someone I was cheering for got the ball.

The same can be seen in animals, as illustrated in an old black-and-white photograph from Wolfgang Köhler’s classic tool-use studies on chimpanzees. One ape, Grande, stands on boxes that she has stacked up to reach bananas hung from the ceiling, while Sultan watches intently. Even though Sultan sits at a distance, he raises his arm in precise synchrony with Grande’s grasping movement. Another example comes from a chimpanzee filmed while using a heavy rock as a hammer to crack nuts. The actor is being observed by a younger ape, who swings his own (empty) hand down in sync every time the first one strikes the nut. Body mapping provides a great shortcut to imitation.

When I see synchrony and mimicry—whether it concerns yawning, laughing, dancing, or aping—I see social connection and bonding. I see an old herd instinct that has been taken up a notch. It goes beyond the tendency of a mass of individuals galloping in the same direction, crossing the river at the same time. The new level requires that one pay better attention to what others do and absorb how they do it. For example, I knew an old monkey matriarch with a curious drinking style. Instead of the typical slurping with her lips from the surface, she’d dip her entire underarm in the water, then lick the hair on her arm. Her children started doing the same, and then her grandchildren. The entire family was easy to recognize.

There is also the case of a male chimpanzee who had injured his fingers in a fight and hobbled around leaning on a bent wrist instead of his knuckles. Soon all of the young chimpanzees in the colony were walking the same way in single file behind the unlucky male. Like chameleons changing their color to match the environment, primates automatically copy their surroundings.

When I was a boy, my friends in the south of the Netherlands always ridiculed me when I came home from vacations in the north, where I played with boys from Amsterdam. They told me that I talked funny. Unconsciously, I’d return speaking a poor imitation of the harsh northern accent.

The way our bodies—including voice, mood, posture, and so on—are influenced by surrounding bodies is one of the mysteries of human existence, but one that provides the glue that holds entire societies together. It’s also one of the most underestimated phenomena, especially in disciplines that view humans as rational decisionmakers. Instead of each individual independently weighing the pros and cons of his or her own actions, we occupy nodes within a tight network that connects all of us in both body and mind.